1. Field of the Invention
This invention relates to a laser source and more particularly to a modulated laser source suitable for optical storage applications.
2. Prior Art
Laser sources have long been regarded as the primary light sources for optical storage applications. For such applications the laser source should be compact, reliable, low cost and have sufficient power for high data rate recording and reading of data on an optical storage medium. In addition, a short wavelength source is needed to achieve the highest possible storage density.
To date, short wavelength laser sources that have sufficient power to produce the required high data rates have neither had a modulation rate sufficiently high for the optical storage application nor control by voltages approaching the logic level voltages which are preferred for an optical storage system.
One potential means of producing short wavelength laser radiation is to use nonlinear optical processes such as frequency doubling to generate higher harmonics of existing practical laser sources.
U.S. Pat. No. 4,510,402 to Summers et al discloses a harmonic generator comprising a pair of uniaxial birefringent KDP crystal elements. The crystal elements are fixed in a position in a series arrangement which provides either a linearly or elliptically polarized second harmonic wave. The "e" directions of the crystal elements are oriented in quadrature.
The publication "Electro-Optic Harmonic Conversion Switch for Large-Aperture Multipass Laser Systems" by Henesian et al, Optics Letters, Vol. 9, No. 8, August, 1984 pp. 365-7 discusses a switched second harmonic generation system for high-power laser systems. This system utilizes a KDP crystal, and a voltage of 52KV is applied to electrodes arranged longitudinally on the crystal to produce the switching of the fundamental beam to the second harmonic beam.
U.S. Pat. No. 4,314,210 to Everett shows an optical system comprising an electro-optic modulator, KDP, which can be positioned within a laser cavity and energized with a periodically varying signal superimposed on a ramp voltage to produce both mode-locking and chirping of the laser output. The ramp voltage amplitude is several thousand volts.
Everett shows a crystal within a laser cavity but he does not suggest second harmonic generation. Both Summers and Henesian show second harmonic generation, but there is no suggestion that the crystal could be located within the laser cavity. All of the references require high control voltages which are not compatible with the high data rates required for optical storage applications, and, in addition, none of the references suggest that the crystal could be included as part of an electrical stripline circuit.